PROJECT SUMMARY/ABSTRACT Functional Communication Training (FCT) is the most widely used treatment approach to address problem behavior exhibited by individuals with developmental disabilities. This strategy has been demonstrated to be effective in numerous published studies. However, little research has been conducted to identify treatment components that may influence the persistence of appropriate communicative behavior and relapse of problem behavior when implementation of FCT is challenged. It is possible that the amount and type of reinforcement provided for communicative behavior during intervention and the frequency with which the communicative behavior and problem behavior encounter extinction during intervention may impact the maintenance of treatment gains when treatment integrity is challenged (e.g., treatment is not implemented as designed). Our previous research on FCT maintenance has yielded several findings, two of which are particularly relevant to the current project. First, response variables such as preference for communication strategy can affect the persistence of communicative behavior. Second, when FCT is challenged with extinction, problem behavior often, though not always, re-emerges. Reinforcement related variables have been demonstrated to impact response persistence in the basic behavioral momentum theory literature and in the applied literature with respect to the persistence of problem behavior. What is less well understood is how reinforcement related variables impact appropriate behavior taught in the context of treatment to address clinically relevant concerns. Similarly, the relapse of previously extinguished behavior has been demonstrated to occur in the basic and applied literature. Reinforcement related variables have been implicated in the level of relapse observed in these studies. Less is known regarding the role of extinction during alternative reinforcement schedules (such as FCT) plays in promoting or mitigating persistence of appropriate responses and relapse of problem behavior. Clinically, it is important that we design treatments that not only result in the reduction of problem behavior and the development of appropriate strategies to obtain reinforcers, but that also produce appropriate behavior that is resilient in the face of challenges to treatment and limit the relapse of problem behavior during those challenges. The proposed studies will provide more information regarding how to program reinforcement for communication strategies such that those responses will persist longest when FCT is challenged, and will begin to clarify the role extinction plays in the maintenance of FCT treatment effects so that treatment can be designed to minimize relapse.